In bone tissue engineering a biodegradable scaffold serves as a temporary substrate, inserted into the sites of defective or lost bone, to support and stimulate bone tissue regeneration. The scaffold gradually degrades and is replaced by new bone tissue (Persidis, A., Nat. Biotechnol. 17:508-10, 1999; Service, R. F., Science 289:1498-1500, 2000; Petite, H., et al., Nat. Biotechnol. 18:959-63, 2000). Ceramics and polymers, referred to as bioceramics and biopolymers, respectively, have been developed for use as tissue engineering scaffolds. Bioceramics have a chemical composition resembling that of natural bone, and allow the growth of bone (a process called osteogenesis) to occur (Jarcho, M., Clin. Orthop., 259-78, 1981; Hench, L. L. and Wilson, J., Science 226:630-6, 1984).
Despite their favorable biological properties, bioceramics are inherently brittle and have low biodegradation rates, which severely limits their clinical use. Biopolymers, on the other hand, have some distinct advantages over ceramic materials. Their biodegradation rates and mechanical properties can be tailored to a certain extent for specific applications. They are particularly amenable for implantation and can be easily manufactured into desired shapes (Yang, S., et al., Tissue Eng. 7:679-89, 2001; Niklason, L. E., Nat. Biotechnol. 18:929-30, 2000). The major concern associated with polymer scaffolds is low mechanical strength and inability to retain their shape.
A number of natural and synthetic polymers have been studied for use as scaffolds. The typical synthetic polymers include poly(glycolic acid) (PGA), poly(L-lactic acid) (PLLA) (Saito, N., et al., Nat. Biotechnol. 19:332-5, 2001; Chen, G., et al., J. Biomed. Mater. Res. 57:8-14, 2001; Andriano, K. P., et al., J. Biomed. Mater. Res. 48:602-12, 1999) and their copolymer, for example, poly(DL-lactic-co-glycolic acid) (PLGA) (Yang, S., et al., Tissue Eng. 7:679-89, 2001). These synthetic polymers demonstrate insufficient cell adhesion, however, and their surfaces are hydrophobic, hindering cell growth in a three-dimensional architecture (Chen, G., et al., J. Biomed. Mater. Res. 51:273-9, 2000; Lahiji, A., et al., J. Biomed. Mater. Res. 51:586-95, 2000). They also lack functional groups available for further surface modifications (Thomson, R. C., et al., J. Biomater. Sci. Polym. Ed. 7:23-38, 1995). When implanted in vivo, at least some synthetic biopolymers release acidic degradation products and invoke a chronic immune reaction that is harmful to host tissues (Daniels, A. U., et al., J. Appl. Biomater. 5:51-64, 1994). In addition, certain bulk hydrolyzing PLGA copolymers have been shown to significantly reduce osteogenesis in healing bone (Martin, C., et al., Biomaterials 17:2373-80, 1996).
Chitosan, a natural cationic polymer, is biologically renewable, biodegradable, biocompatible, non-antigenic, and non-toxic. Chitosan structures have a hydrophilic surface that promotes cell adhesion, proliferation and differentiation, and evokes a minimal foreign body reaction on implantation (Suh, J. K. and Matthew, H. W., Biomaterials 21:2589-98, 2000; Hutmacher, D. W., et al., Ann. Acad. Med. Singapore 30:183-91, 2001). Chitosan scaffolds promote the growth of bone cells within the scaffolds, and can enhance bone formation both in vitro and in vivo (Muzzarelli, R. A., et al., Biomaterials 15:1075-81, 1994). In spite of its general acceptance as a material that is compatible with living tissue, chitosan is mechanically weak and unstable, and unable to maintain a predefined shape for transplantation as a result of swelling (Shanmugasundaram, N., et al., Biomaterials 22:1943-51, 2001). Alginate, an anionic polymer, widely used as an instant gel for bone tissue engineering (Lauffenburger, D. A. and Schaffer, D. V., Nat. Med. 5:733-4, 1999), is biocompatible, hydrophilic, and biodegradable under normal physiological conditions (Gutowska, A., et al., Anat. Rec. 263:342-9, 2001).
The present invention provides biodegradable porous structures made from chitosan and alginate. The compressive strength of the structures, and other physical properties, such as their elasticity, ability to retain their shape, and ability to promote the growth of bone-forming cells, facilitate the use of the structures as substrates to support the growth of bone in vivo or in vitro. Additionally, the porous structures of the invention can also be used as substrates for the growth of cells that form cartilage.